The present invention relates to a high frequency heating apparatus that performs induction heating using a magnetron, for example a microwave-cooking oven; more specifically to the structure of a circuit for driving a magnetron.
The power supply circuit incorporated in a home-use high frequency heating apparatus and various other such apparatus is bulky and heavy in weight. Reduction in size and weight of the power supply circuit has been one of the main tasks in the industry. Efforts have been made in many sectors to make it compact, light and inexpensive through adoption of the switching power supply. Also in the field of food processing high frequency heating apparatus using a magnetron, the magnetron driving circuit is requested to reduce its size and weight. There is a patent that intends to meet the requirement through introduction of the switching power supply (PCT/JP98/00751).
According to the above patent, the switching loss of semiconductor switching devices operating at high frequency is reduced through adoption of a resonance type circuit, which circuit being an essential technology in the switching power supply. Because of a high voltage in the circuit generated by the functioning of resonance circuit, the semiconductor switching devices and other relevant electric components are required to have high voltage specifications. This makes the circuit large and heavy. In order to evade such problems the above patent discloses the following structure.
As shown in FIG. 28, a conventional circuit comprises a DC source 1, a leakage transformer 2 connected to one end of the DC source 1, a first semiconductor switching device 6 connected in series to a primary coil 3 of the leakage transformer 2 and to the other end of the DC source, a first capacitor 4 connected in parallel with the primary coil 3 of the leakage transformer 2, a series circuit of a second capacitor 5 and a second semiconductor switching device 7, driving means 8 having an oscillator for driving the first semiconductor switching device 6 and the second semiconductor switching device 7, rectifying means 10 connected to a secondary coil 9 of the leakage transformer 2, and a magnetron 11 connected to the rectifying means 10. The series circuit of the second capacitor 5 and the second semiconductor switching device 7 is connected in parallel with the primary coil 3 of the leakage transformer 2.
A feature of the above circuit structure is that a voltage to be applied on the main first semiconductor switching device 6 can be lowered by the use of an auxiliary second capacitor 5 that has a capacitance greater than that of the first capacitor 4 which forms a resonance circuit in combination with the leakage transformer 2.
Thinking of cases where the DC source 1 is provided by rectifying a commercial power supply. The commercial power supply comes in different voltages, for example 100V in Japan, 120V in the U.S., 240V in the UK and Peoples Republic of China, 220V in Germany. Even within Japan, many of the power consuming professional apparatus receive the power supply at 200V.
In a case where the commercial supply voltage is 100V or 120V, the voltage to be applied on the main first switching device 6 may be reduced by the above described circuit. However, if voltage of the commercial power supply is higher than 200V, the voltage to the main first switching device 6 goes high in the circuit disclosed by the above patent. Also, it is necessary to change inductance of the primary coil and the secondary coil of leakage transformer 2, as well as capacitance of the first capacitor 4 and the second capacitor 5.
Table 1 compares the constants of the leakage transformer 2, the first capacitor 4 and the second capacitor 5, and the voltages on the first switching device 6 in the 100V power supply and the 200V power supply. Table 1 teaches us, for example, that the inductance of primary coil of leakage transformer 2 increases to approximately 4 times, the number of coil turns increases, in proportion to the square root of inductance, to approximately two times. Thus the structure undergoes a substantial change.
Also, the withstand voltage of the first switching device 6 has to be raised to meet the two-fold voltage to be applied thereon.
The conventional circuit has two problems when it encounters a commercial power supply higher than 200V. One problem is that the voltage applied on the switching device goes high, the other problem is that the leakage transformer, the first capacitor and the second capacitor are compelled to have different constants respectively.
The conventional circuit is described further referring to FIG. 28. A parallel resonance circuit formed of the leakage transformer 2, the first capacitor 4 and the second capacitor 5 makes the voltage of primary coil 3 to be higher than the DC source voltage by the resonance effect.
Therefore, when the DC source is provided from a high voltage commercial power supply, the voltage of primary coil 3 goes still higher. So, it becomes necessary to lower the step-up ratio of leakage transformer 2 (ratio in the number of turns between the primary coil 3 and the secondary coil 9), and to increase the number of turns of primary coil 3 in order to lower the voltage of primary coil 3.
The DC source 1 is composed of rectifying means for rectifying commercial power supply and a filter formed of an inductor and a capacitor to smooth the output.
The filter smoothes the voltage, removes a noise generated as a result of switching operation by switching device and avoids intrusion of a noise from outside.
However, the filter formed of an inductor and a capacitor generates an overvoltage twice as high as that of the DC source at the moment when the power is on.
There is another problem that is related with a sudden change of impedance caused by a discharge started within magnetron tube. Relationship between primary coil 3 and secondary coil 9 of the leakage transformer 2 is shown in FIG. 32(a), in the form of an equivalent circuit diagram. The primary coil 3 may be divided into a leakage inductor and an exciting inductor; further the exciting inductor and the secondary coil 9 magnetically coupled with an ideal transformer (magnetic coupling coefficient 1). Both ends of the secondary coil are connected with the rectifier circuit, the rectifier circuit is connected to a magnetron. In the drawing, L1 represents inductance of the primary coil, L2 inductance of the secondary coil.
When impedance of a magnetron becomes extremely small to an equivalence of short-circuiting of the secondary coil, an equivalent circuit of leakage transformer 2 is as shown in FIG. 32(b). There is only a leakage inductor. The inductance L of which is given by the formula below.
L=(1xe2x88x92k2)xc3x97L1xe2x80x83xe2x80x83(Formula 1)
where:
L1 is inductance of primary coil
k is coefficient of magnetic coupling between primary coil and secondary coil
When the secondary coil 9 is short-circuited, inductance of primary coil becomes small. Therefore, a large current as shown in Formula 2 flows to the first switching device 6.
Ic=VDCxc3x97Ton/L (1)xe2x80x83xe2x80x83(Formula 2)
VDC: output voltage of DC source 1
Ton: Time of conduction in the first switching device 4
Because of the small L the current is an overcurrent; furthermore, an overvoltage is generated when the first switching device 6 turns OFF. Thus the first switching device 6 is given with a great stress by the overcurrent and the overvoltage emerging continuously.
Still, there is a following problem with respect to driving of a magnetron. An appropriate temperature for magnetron cathode is approximately 2100xc2x0 K. If the cathode temperature is not appropriate, the magnetron is unable to operate and impedance between anode and cathode is greater then 100Mxe2x96xa1. When a cathode is in an appropriate temperature, the constant voltage characteristics of magnetron keeps the voltage between anode and cathode at xe2x88x924 kV. As the cathode heating current is delivered from a tertiary coil 26 of leakage transformer, as shown in FIG. 28, a state of high impedance between the anode and the cathode at the start-up lasts until the cathode temperature reaches the appropriate level. During the state, the constant voltage characteristics do not exist between the anode and the cathode, and a voltage higher than xe2x88x924 kV is generated.
The rectifier and the filter forming the DC source 1, the first switching device 6, the second switching device 7, the first capacitor 4, the second capacitor 5, the driving means 8, the leakage transformer 2 and the rectifying means 10 are normally disposed on a same printed circuit board for the sake of compactness. FIG. 29 shows structure of a printed circuit board 12. The rectifying means 10 is connected with the secondary coil 9 of leakage transformer 2, while the other electric components are connected with the primary coil 3; and the former and the latter are insulated with each other.
However, if some of these components are mounted aslant or they are tilted later by an external force, there is a possibility that the electric components forming the rectifying means 10 and those connected with the primary coil 3 of leakage transformer 2 might come into contact with each other. In order to avoid such a contact to happen, following means are taken.
As illustrated in FIG. 29, a high withstand voltage diode 13, which being an electric component pertaining to the rectifying means 10, is disposed away from the first capacitor 4, with a distance long enough not to cause mutual contact even if they topple.
The electric components to be connected with the primary coil of leakage transformer 2 are molded, in order that they do not fall down even if a certain external force is given to. Or, a barrier is provided between components for supporting a toppled component, if any, in order that it does not fall down among the other components. The electric components are mounted and fixed with glue. These are some of the methods taken in the conventional configuration to avoid the troubles.
Conventional measures taken to prevent electric components on a printed circuit board from making mutual contact by an accidental force, or to prevent those components that were mounted oblique by mistake of an assembly person from making mutual contact at a later stage, have following drawbacks. Problem 1; mounting the components on a board with a sufficient inter-space enough to avoid mutual contacting even if they fall down takes too much board space, makes it difficult to reduce the board size. Problem 2; molding the components for higher insulating capability means an extra cost needed for providing the additional processing on finished components. Problem 3; providing an insulation board between the high voltage circuit and the low voltage circuit makes assembly of printed circuit board difficult, hence a higher cost. Problem 4; fixing the components by gluing takes longer assembly work hour, and effectiveness of the gluing does not necessarily last long.
A magnetron is disposed in a place apart from a printed circuit board on which electric components for the driving circuit are mounted. So the anode and the cathode of a magnetron need to be connected with the printed circuit board by some means. Cathode is connected to the printed circuit board with a lead wire, while anode is connected to the printed circuit board through a metal cabinet of the high frequency heating apparatus.
As the body itself of a magnetron forms the anode, the electrical connection between anode and metal cabinet is completed by simply mounting and fastening a magnetron on the metal cabinet using a screw bolt.
Now, the electrical connection between the metal cabinet and the printed circuit board is described. The connection is a critical point. If by some reason the connection in this respect was forgotten in the assembly line, magnetron does not operate. Or, if the connection was incompletely made, a resulting contact resistance may generate heat, giving damage on the neighboring components, or in the worst case, the circuit board may be scorched.
The connecting means is described referring to the drawings. In FIG. 28, a first connection point 14 represents a point of connection between a pattern 16 of printed circuit board 12 (FIG. 30) and a point 14xe2x80x2 which signifies metal cabinet of a high frequency heating apparatus, or a metal sheet 17 forming the cabinet (FIG. 30). FIG. 30 is a cross sectional view showing a connecting section of metal cabinet of a high frequency heating apparatus, or a metal sheet 17 forming the cabinet, and the printed circuit board 12. A cylindrical eyelet 18 inserted to a hole of printed circuit board 12 is fixed by solder 15 with a pattern 16 of the printed circuit board 12 to share a same electrical potential. The hole diameters of the eyelet 18 and the hole in the metal sheet are slightly smaller than the diameter of a tapping bolt 19. When the tapping bolt 19 is screwed tight, it proceeds cutting into part of the eyelet 18 and the metal sheet 17, and the solder 15 is fixed firmly onto the metal sheet 17. Thus, the pattern 16 and the metal sheet 17 have a sure electrical connection with each other.
FIG. 31 shows the printed circuit board 12 mounted with electric components, as viewed from the top. The printed circuit board 12 is provided with as many as five holes 20, 21, 22, 23 and 24 in the edge portion for fixing, because it bears on it relatively heavy substance such as the leakage transformer. And a resin frame is provided for protecting the printed circuit board from a damage that could be caused by vibration, shock by a drop, etc. during transportation. The above described hole with eyelet is indicated by numeral 25.
A practical weak point with the conventional assembly operation of printed circuit board is that, because there are a plurality of holes to be fastened with screw bolt in a board, an assembly person on the production line might forget to put a screw bolt in the functionally important hole provided with eyelet.
A circuit in accordance with the present invention comprises a DC source, a leakage transformer connected to the positive terminal of the DC source, a second capacitor connected in series to primary coil of the leakage transformer, a second switching device connected to the positive terminal of the DC source, a first capacitor connected in parallel with the second switching device, a first switching device connected in series to the second switching device and connected to the negative terminal of the DC source, driving means for driving the first and the second switching device, rectifying means connected to the secondary coil of the leakage transformer, and a magnetron connected to the rectifying means.
The series connection of primary coil of the leakage transformer and the second capacitor divides the DC source voltage, leading to reduced voltage to be applied on the first and second semiconductor switching devices. And, the circuit of the present invention may use a leakage transformer, a first capacitor and a second capacitor whose respective constants are substantially identical to those in conventional circuits.
A surge absorber provided at the output of a filter forming the DC source absorbs an overvoltage. As the overvoltage is a transient phenomenon and the output voltage of filter quickly returns to the steady voltage (DC source voltage), the surge absorber does not keep absorbing the overvoltage.
Other exemplary structure the circuit is connecting a point of connection between primary coil of the leakage transformer and the second capacitor to one end of the DC source with a first resistor, and the point of connection to with the other end of the DC source with a second resistor for detecting the voltage of primary coil of the leakage transformer. Based on the detection, generation of overvoltage at the start-up in the secondary coil of the leakage transformer can be controlled.
Furthermore, in order to prevent electric components on a printed circuit board from falling down to make contact with other electric components, an anti-toppling tool may be provided at the foot of the electric components. With the above described setup, it becomes unnecessary to provide additional treatment on finished components or extra inter-space between the components, as practiced in conventional circuits. Therefore, a printed circuit board can be made smaller and cheaper.